This application is based on Japanese Patent Application No. 2009-058007 filed with the Japan Patent Office on Mar. 11, 2009, the entire content of which is hereby incorporated by reference.
1. Technical Field
The present invention relates to a device constituting a network and a network system, more particularly to an improved technology concerting supply of electric power to the device through the network.
2. Related Art
PoE (Power over Ethernet (registered trademark)) is a technology for supplying the electric power by utilizing an Ethernet (registered trademark) cable. The use of PoE can facilitate the supply of electric power to the device installed in the site the electric power is hardly directly supplied. In PoE, because both the data and the electric power can be transmitted through the cable, it is not necessary to separately wire the cable for data communication and the cable for supply of electric power, and the wiring is easy to provide when devices constituting the network system are installed in the field.
In the network system such as PoE having the power feeding function, there has been proposed a technology for supplying part of the electric power that is received through the network by the device connected to the network to another device through the network (for example, see Japanese Unexamined Patent Publication No. 2005-269890).
However, for the device having the function of supplying part of the electric power received through the network to another device, a communication port used for power receiving and a communication port are used while previously determined. Unfortunately configuration of the communication port becomes troublesome because the function of each communication port is previously determined. Further, the supply of electric power cannot be received by utilizing the power feeding communication port, even if the communication port configured for the power feeding is vacant in case of trouble with the power receiving communication port or the network cable connected to the power receiving communication port. As disclosed in Japanese Unexamined Patent Publication No. 2005-269890, a connection mode of the devices is limited to the network configuration in which the electric power is supplied from an upstream side to a downstream side like a serial configuration, a hub and spoke configuration, and a combination thereof. Therefore, the technology disclosed in Japanese Unexamined Patent Publication No. 2005-269890 is hardly applied to a network system such as a ring network configuration in which the electric power is probably received from any one of plural communication ports.